In a power plant having a steam generator, the energy content of a fuel is used to evaporate flow medium in the steam generator. The steam generator comprises evaporator tubes to evaporate the flow medium, the heating of which results in evaporation of the flow medium conveyed therein. The steam provided by the steam generator can in turn be provided for instance for a connected external process or however to drive a steam turbine. If the steam drives a steam turbine, a generator or a work machine is usually operated by way of the turbine shaft of the steam turbine. In the case of a generator, the current generated by the generator can be provided to supply a grid and/or isolated network.
The steam generator can be embodied here as a continuous steam generator. A continuous steam generator is known from the paper “Verdampferkonzepte fuer BENSON-Dampferzeuger” [Evaporator concepts for Benson steam generators], by J. Franke, W. Köhler and E. Wittchow, published in VGB-Kraftwerkstechnik 73 [VGB power plant technology 73] (1993), issue 4, pages 352 to 360. In the case of a continuous steam generator, the heating of steam generator tubes provided as evaporator tubes results in evaporation of the flow medium into the steam generator tubes in one single passage.
To achieve a particularly high degree of efficiency in the continuous steam generator, superheater tubes are arranged downstream of the evaporator tubes on the flow medium side, said superheater tubes further increasing the enthalpy of the escaping steam. The superheater tubes are configured for the passage of steam and may be damaged upon the ingress of water. A water-steam separation device is therefore usually arranged upstream thereof on the flow medium side, and may include for instance the water steam separator and a water bottle, the so-called water collecting vessel or combinations comprising separators and water bottles. The water-steam separation device does not completely separate evaporated water from steam, initially collects it and then outputs it via a discharge valve. The separated water can either be discarded or fed into the circuit again for renewed evaporation.
In the water-steam separation device, comparatively little or no water at all flows during the permanent operating state of the continuous steam generator, since the water pumped into the evaporator tubes practically completely evaporates. By contrast, a considerably larger water quantity flows into the water-steam separation device during the starting process. When starting a continuous steam generator, an evaporator minimum mass flow is namely usually initially passed through the evaporator tubes for reasons of adequate tube cooling and the burner is ignited with a partial load. Before commencement of evaporation, the entire water flow is fed here to the water-steam separation device. Upon the onset of the evaporation, one portion of the water content between the site of the start of the evaporation and the water-steam separation device is discharged as a result of the sudden increase in volume conditional thereupon. In order, despite this water discharge, to largely prevent an unwanted routing of unevaporated flow medium into the superheater tubes arranged downstream thereof, a correspondingly large dimensioning of all components of the water-steam separation device and the downstream water supply device (for instance flash trap, capacitor, discharge pipe etc.) is usually necessary, this being associated with a high material outlay and expenditure.
A method for starting a continuous steam generator, with which the water discharge can be avoided or kept to a minimum, is known from DE 19528438. With this method, the ratio of firing power and feed water flow is adjusted such that the water pumped into the evaporator tubes also completely evaporates in the partial load region and thus no or almost no water ingress into the water-steam separation device or the superheater tubes takes place. The water discharge is thus minimized here by a feed water supply which is kept correspondingly low.
However, in the case of continuous steam generators, as described in DE 195 28 438, a minimum mass flow density and thus a minimum feed water mass flow is also needed for the reliable cooling of the evaporator tubes even in the case of a minimal firing power. A reduction in the feed water mass flow to prevent a water discharge is correspondingly not possible.